Refrigeration and air conditioning systems generally include a compressor, a condenser, an expansion valve or equivalent, and an evaporator. These components are coupled in sequence in a continuous flow path. A working fluid flows through the system and alternates between a liquid phase and a vapor or gaseous phase.
A variety of compressor types have been used in refrigeration systems, including but not limited to reciprocating compressors, screw compressors and rotary compressors. Rotary compressors can include the vane type compressors as well as the scroll machines. Scroll compressors are constructed using two scroll members with each scroll member having an end plate and a spiral wrap. The spiral wraps are arranged in an opposing manner with the two spiral wraps being interfitted. The scroll members are mounted so that they may engage in relative orbiting motion with respect to each other. During this orbiting movement, the spiral wraps define a successive series of enclosed spaces, each of which progressively decreases in size as it moves inwardly from a radially outer position at a relatively low suction pressure to a central position at a relatively high pressure. The compressed gas exits from the enclosed space at the central position through a discharge passage formed through the end plate of one of the scroll members.
Under any one of a number of adverse conditions, the discharge gas of the scroll compressor can become excessively hot, which in turn can adversely effect the efficiency as well as the durability of the compressor. One known prior art method of cooling the compressed gas is to inject liquid refrigerant from the condenser through an injection passage directly into the compressor. The liquid refrigerant may be injected into the suction gas area of the compressor or it may be injected into an intermediate enclosed space defined by the scroll members. These various methods are shown in U.S. Pat. No. 5,076,067, U.S. Pat. No. 4,974,427, U.S. Pat. No. 5,329,788 and U.S. patent application Ser. No. 08/237,449, filed May 3, 1994, entitled "Scroll Compressor With Liquid Injection", all of which are assigned to the same assignee as the present application, the disclosure of each of which is hereby incorporated herein by reference. It is desirable for optimum operating efficiency and effective cooling of the discharge gas that the liquid injection port be located as centrally or as close to the discharge passage, as is possible. Unfortunately, however, the centralized location of the injection port is limited by the liquid supply pressure at the outlet of the condenser, which is near the discharge pressure but still intermediate the suction pressure and the discharge pressure of the compressor. If the pressure of the gas in an enclosed space near the discharge port is greater than the condenser outlet liquid supply pressure throughout an entire cycle of orbiting motion, the liquid refrigerant cannot flow from the liquid injection passage into the enclosed space in the compressor.
It is therefore desirable either to lower the pressure of the central or innermost enclosed space to a pressure which is below the liquid supply pressure during at least a portion of the cycle of orbiting movement or to inject the liquid remotely enough from the discharge area so that the above stated constraints under space pressures are met. This lowering of the pressure will enable positive liquid injection through an injection port which can be located closer to the discharge port which is where the compressed refrigerant is the hottest and thus where cooling is most effective. One method of lowering the pressure in the central innermost enclosed chamber is the use of a dynamic one-way valve in the discharge passage which opens and closes once every cycle. Such one-way valves, however, can be noisy, have potential reliability implications, and they reduce compressor efficiency due to gas flow loss. In addition, these one-way valves require additional costs for the extra components, as well as additional costs for their assembly.
Additionally, some prior art designs of liquid injection systems utilize a solenoid valve for selectively blocking the flow of liquid refrigerant to the compressor when the refrigeration cycle is turned off. The purpose of these solenoid valves is to allow liquid injection while the compressor is running and to prevent the flow of refrigerant from the condenser to the enclosed spaces during times when the compressor is shut down, thus avoiding compressor flooding which can be the cause of severe damage due to liquid refrigerant slugging upon compressor startup. However, solenoids valves that are wired so that they may be opened when the compressor is energized can present problems under certain circumstances. When the compressor overheats, an internal temperature sensor cuts the electrical power to the motor. When this occurs, the solenoid valve may still be powered and thus the compressor will no longer operate but the valve will still be open allowing the liquid refrigerant to be bled to the compressor's enclosed spaces. The abundant presence of liquid refrigerant in the enclosed spaces will cause flooded starts upon re-start of the compressor, as stated above. Electrical means to prevent such situations are possible. For example, wiring the solenoid valve in series with the motor windings can provide such protection. Other alternatives include the sensing of the current in the motor winding and closing the valve when a current absence condition is sensed.
Accordingly, the continued development of liquid injection systems is directed to a low cost system which can position an injection port closer to the discharge passageway as well as being able to eliminate any problems encountered during startup of the compressor. The present invention provides the art with an injection system which utilizes an internal pilot valve to control the liquid injection system. The internal pilot valve operates in response to discharge pressure and will only allow liquid injection at a time when the discharge pressure is above a specified minimum.
Other advantages and objects of the present invention will become apparent to those skilled in the art from the subsequent detailed description, appended claims and drawings.